Mechanisms of Formation and Thermal Stabililty of Functional Nanostructures
There are many challenges in materializing the applications utilizing inorganic nanoparticles. The primary drawback is the degradation of properties due to aggregation and sintering either due to elevated temperatures or prevailing chemical/electrochemical conditions. In this thesis, various wet chemical synthesis methods are developed to obtain metal nanostructures with enhanced thermal stability. The thesis is organized as below: Chapter 1 presents the problems and challenges in materializing the application of nanomaterials associated with the thermal stability of nanomaterials. A review of the existing techniques to improve the thermal stability and the scope of the thesis are presented. Chapter 2 gives a summary of the various materials synthesized, the method adopted for the synthesis and the characterization techniques used in the material characterization. Chapter 3 presents a general template-less strategy for the synthesis of nanoporous alloy aggregates by controlled aggregation of nanoparticles in the solution phase with excellent control over morphology and composition as illustrated using PdPt and PtRu systems as examples. The Pt-based nanoporous clusters exhibit excellent activity for methanol oxidation with good long term stability and CO tolerance. Chapter 4 presents a detailed study on the thermal stability of spherical mesoporous aggregates consisting of nanoparticles. The thermal stability study leads to a general conclusion that nanoporous structures transform to hollow structures on heating to elevated temperatures before undergoing complete densification. Chapter 5 presents a simple and facile method for the synthesis of single crystalline intermetallic PtBi hollow nanoparticles. A mechanism is proposed for the formation of intermetallic PtBi hollow structures. The intermetallic PtBi hollow structures synthesised show excellent electrocatalytic activity for formic acid oxidation reaction. Chapter 6 presents a robust strategy for obtaining a high dispersion of ultrafine Pt and PtRu nanoparticles on graphene. The method involves the nucleation of a metal precursor phase on graphite oxide surfaces and subsequent reduction with a strong reducing agent. The electrocatalytic activity of the composites is investigated for methanol oxidation reaction. Chapter 7 presents a microwave-assisted synthesis method for selective heterogeneous nucleation of metal nanoparticles on oxide supports leading to the synthesis of high activity catalysts. The catalytic activity of the hybrids synthesized by this method for investigated for H2 combustion. Chapter 8 presents thermal stability studies carried out on nanostructures by in-situ heating in transmission electron microscope. The microstructural changes during the sintering process are observed in real time and the observations lead to the understanding of the mechanism of particle growth and sintering. At the end, the results of the investigations were summarized with conclusions drawn.
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